Density-enhanced remediation of non-aqueous phase liquid contamination of subsurface environments

ABSTRACT

A method for remedying contamination of a subsurface environment by a non-aqueous phase liquid that is denser than water (DNAPL). The subsurface environment has a resident aqueous phase and a DNAPL phase. A dense aqueous solution that has a density greater than a density of the DNAPL phase is introduced to the subsurface environment. The dense aqueous solution displaces the resident aqueous phase and causes the DNAPL phase to rise above the greater density aqueous solution. The DNAPL phase is then recovered, which substantially remedies the contamination of the subsurface environment. An alternative method provides for introducing a dense solution layer that has a density greater than the density of the DNAPL phase beneath the subsurface region contaminated with the DNAPL phase. Next, the water table is lowered so as to create an unsaturated porous medium above the dense solution layer and to downwardly mobilize the DNAPL phase to the top of the dense solution layer. The DNAPL phase is then collected from the top of the dense solution layer.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/198,988 filed Nov. 23, 1998 now U.S. Pat. No. 6,190,092.

GRANT STATEMENT

This invention was made in part from government support under GrantNumber 5 P42 ES005948 from the National Institute of EnvironmentalHealth Sciences (NIEHS), a branch of the National Institutes of Health(NIH). The U.S. Government has certain rights in the invention.

TECHNICAL FIELD

The present invention relates generally to methods for remedyingsubsurface soil contamination. More particularly, the present inventionrelates to a method for remedying the contamination of a subsurfaceenvironment by dense non-aqueous phase liquids (DNAPLs).

The publications and other materials used herein to illuminate thebackground of the invention, and in particular cases, to provideadditional details respecting the practice, are incorporated herein byreference, and for convenience, are referenced by author and date in thefollowing text, and respectively grouped in the appended list ofreferences.

Table of Abbreviations CF cosolvent flushing DERD density-enhancedremediation of DNAPLs DNAPL dense non-aqueous phase liquids IBD In situbiodegradation IFT interfacial tension ISGS in situ gas stripping LNAPLless dense non-aqueous phase liquids NAPL non-aqueous phase liquids PATpump-and-treat PCE perchloroethylene RW reactive walls IS steaminjection SF surfactant flushing 1,1,1-TCA 1,1,1-trichloroethane1,1,2-TCA 1,1,2-trichloroethane TCE trichloroethylene VE vaporextraction

BACKGROUND ART

Contamination of subsurface environments by fluids that are immisciblewith water has occurred routinely in the United States andindustrialized countries around the world over the last 40 years. Mercerand Cohen (1990). Such fluids are often termed non-aqueous phase liquids(NAPLs) in general, or LNAPLs when less dense or DNAPLs when more densethan the groundwater present in the subsurface. Typical LNAPLs arepetroleum products (e.g., gasoline, diesel fuel, jet fuels, heatingoils), and typical DNAPLs are chlorinated solvents (e.g.,trichloroethylene, tetrachloroethylene, dichloroethanes). Schwille(1988); Bartow and Davenport (1990).

Species present in such NAPL phases can solubilize to the aqueous phase,volatilize to the gas phase, or sorb to the solid phase present in thesubsurface. Environmental concerns result when such species are linkedto human or ecological health concerns and become present in sufficientquantities in a mobile aqueous or gas phase. Such health concerns areassociated with common constituents of NAPLs found routinely in theenvironment, such as trichloroethylene and benzene.

Once released into the subsurface, NAPLs migrate and typically reach astable, immobile state within a relatively short time scale (hours todays) after the source is removed. Immobile NAPLs can remain in thesubsurface over time scales that can range from months to decades orlonger under natural conditions, because they are comprised of speciesthat are only sparingly soluble in water. Miller, Poirier-McNeill et al.(1990). As a result, NAPLs are considered a long-term source ofgroundwater contamination. Mayer and Miller (1996).

DNAPLs tend to be an even more significant problem than LNAPLs becauseof the following characteristics:

(1) they were routinely used in industrial practices, spilled, andintentionally disposed of in the subsurface in the United States,starting in the 1960's and continuing for two decades;

(2) they often migrate larger distances than LNAPLs;

(3) they often penetrate the water table;

(4) they can form pools contained by low-permeability materials;

(5) they are often comprised of species that tend to degrade slowly inmany systems;

(6) they are extremely difficult to locate and remove; and

(7) they contain species that are typically regulated at lowconcentrations in drinking water (e.g., 5 μg/L).

Because of the above characteristics, remediation of subsurfacecontamination resulting from DNAPLs is a frequently encountered problem,which has proven to be extremely difficult. Methods that have been usedto remove such contamination include: pump-and-treat, cosolventflushing, surfactant flushing, steam injection, and in situbiodegradation. To varying extents, all of these strategies have beenimplemented at the laboratory and pilot or full field scale. Each ofthese methods results in the removal of some solute mass from a systemcontaminated with a DNAPL, but the rate at which the removal occurs andthe expense involved with this standard set of methods leaves theproblem of DNAPL remediation unsolved. The advantages and disadvantagesof the common set of DNAPL removal and containment strategies aredescribed in detail as follows.

Pump-and-Treat

Pump-and-treat (PAT) is perhaps the most common method of subsurfacerestoration. The method consists of installing a well in a regioncontaminated by a DNAPL and pumping it, which results in the inductionof a flow of groundwater in all directions toward the extraction wellfor some local volume in the vicinity of the well called the capturezone. de Marsily (1986); Domenico and Schwartz (1990). As thegroundwater passes through a capture zone that is contaminated withDNAPL, a portion of the DNAPL will dissolve into the groundwater and betransported with it. If a PAT scheme is continued for a sufficientperiod of time, all of the DNAPL present in the capture zone will besolubilized and exit the system through the pumping well. Mayer andMiller (1996). In theory, placement of a sufficient number of adequatelydesigned pumping wells and appropriate operation for a sufficient periodof time will result in the removal of all DNAPL contaminants from acontaminated system.

Once these contaminants are brought to the surface from the wells,treatment of the contaminated waste stream is accomplished and the wateris either injected back into the subsurface or disposed of using othermeans. The actual treatment processes used depends upon thecharacteristics of the waste stream, treatment objectives, dischargestream quality constraints, and other factors.

The PAT method is appealing because of its simplicity. However,experience has shown that this approach is not very efficient, whereefficiency is defined as the mass of DNAPL removal per volume of fluidsremoved. Mackay and Cherry (1989). This lack of efficiency results fromthe upper bound on the mass removal rate that results from thesolubility of the DNAPL in the water phase, and because the waterremoved from the pumping wells may be far removed from the solubilitylimit of the DNAPL in the fluid phase. The solubility of the DNAPL formsthe upper bound on the mass removal rate because capillary forces thattrap a DNAPL are typically not overcome by the viscous forces induced bypumping; thus, mobilization of free-phase DNAPL typically does not occurin PAT operations. Further, if mobilization does occur, viscous forcesmust be greater than the sum of both capillary and gravity forces iffree phase capture by a pumping well is to occur. Because efficiency islow for PAT methods and cleanup standards for DNAPLs are oftenstringent, PAT remediation is ineffective as a means of remediatingDNAPL-contaminated groundwater over time scales of even years. For thisreason, PAT methods have been classified in some instances as acontaminant containment strategy, rather than as a restoration method.

PAT methods are inefficient not only because they do not mobilize theDNAPL into a capture well, but also because DNAPL distribution patternsin typical heterogeneous subsurface systems results in the formation ofregions of high saturations of DNAPLs called pools. These pools in turnblock the pore space of porous media, deflecting groundwater flow aroundthe DNAPL zone. Since contaminant removal occurs only through masstransfer from the DNAPL to the aqueous phase, the surface area thatexists between the groundwater and DNAPL, which is relatively small,results in a significant mass transfer limitation for most systems,which is manifest as concentrations of dissolved contaminants that arefar removed from equilibrium values.

Vapor Extraction

Vapor extraction (VE) is a frequently used method of remediation inwhich a well (or wells) are installed into the unsaturated zone and avacuum is applied to the well(s). Walter (1994); U.S. Army Corps ofEngineers (1995). Much like PAT, a pressure gradient is established in alocal area around the well, which results in gas-phase flow toward thewell. If the contaminant of interest is present in the gas phase, thenremoval of the contaminant occurs as the gas phase is removed from thesystem. Because much less energy is required to extract a volume of gasfrom the subsurface compared to an equivalent volume of water, VE hasthe potential to be more efficient than PAT methods. Ross and Lu (1994);Travis and Macinnis (1992).

In order to realize the potential efficiency of VE methods, acontaminant must be present in the gas phase. If the contaminantoriginally enters the subsurface as a NAPL, then volatilization to thegas phase must occur for all species of interest, thus VE is only apotentially viable technology for relatively volatile species. She,Sleep etal. (1995). In addition, volatilization must occur at asufficiently rapid rate such that the gas-phase concentrations aresustained at a relatively high level compared to their vapor pressure.If this does not occur, then mass removal rates are said to be masstransfer limited. VE systems are often mass transfer limited, removingcontaminants with a concentration that is much less than the vaporpressure of the target species.

A more important problem with VE exists for DNAPL contamination thatexists in the saturated zone: a contaminant must undergo a series ofmass transfer steps, first from the DNAPL phase to the water phase andsecond from the water phase to the gas phase. For contaminants to beremoved from the system, they must volatilize from the top of the watertable into the gas phase and be removed in the VE system. Thedissolution of the DNAPL into the aqueous phase and transport tointerface of the unsaturated zone requires a series of physical andchemical transport steps that in sum are usually much slower thatvolatilization of NAPL in the unsaturated zone alone. Additionally VEdoes not aggressively control the movement of a DNAPL in the saturatedzone, where the majority of such contaminants are likely to reside. Forthis reason, PAT and VE are often used together—a procedure thattypically increases the rate at which contaminants are removed from thesubsurface compared to the use of either method alone. However, thisoverall rate of removal is still sufficiently slow as a result of masstransfer limitations that VE is not an efficient means of DNAPLremediation for most sites. Mccann, Boersma et al. (1994); Larkin andHemingway (1991).

An effort to reduce the mass transfer and transport limitations of VEhas resulted in a related process known as in situ gas stripping (ISGS).lSGS is similar to VE except that a gas phase is injected into thesaturated zone. Marley (1991). As the gas phase is transportedvertically under gravity forces through the saturated zone and exitsinto the unsaturated zone, the hope is that an increased rate of DNAPLremoval from the saturated zone results compared to VE. While ISGS canoften be more effective than VE or VE and PAT at removing DNAPLcontaminants from the saturated zone, an injected gas phase oftenfollows preferential pathways under gravity-dominated forces anddirectly influences a volume within a relatively small distance of thegas-phase injection well.

In Situ Biodegradation

In situ biodegradation (IBD) is a process that encourages theestablishment and growth of microorganisms that will transformcontaminants originally present in a DNAPL into products that pose lessof risk than the original contaminant. The reactants and products,microorganisms required, and relative rates at which suchtransformations occur depend upon the contaminant of concern, theelectron acceptor conditions, and a variety of other conditions.Domenico and Schwartz (1990), Dupont (1993). Determining specificpathways, rates, and conditions that encourage such transformations fora wide variety of contaminants is an active area of research.

In cases in which contaminants are transformed quickly into inoculousproducts, IBD can be an attractive process. However, many DNAPLs arecomprised of species for which rates of transformation have been foundto be relatively slow, especially for native microbial populations andredox conditions. For example, contaminants such as trichloroethylene(TCE) and perchloroethylene (PCE) have proven relatively resistant torapid microbial transformations. Tsien, Brusseau et al. 1989; Semprini,Roberts et al. (1990). An additional, but familiar, limitation occurswith IBD processes: mass transfer and transport limitations. In orderfor transformations to occur, the contaminant must be dissolved into theaqueous phase and come into contact with an effective microbialpopulation in the presence of needed nutrients and appropriate electronacceptors. Zacharias, Lang et al. (1995). For these reasons, IBD hasnot, in general, proven to be an effective means of remediatingDNAPL-contaminated subsurface systems.

Cosolvent Flushing

Cosolvent flushing (CF) is one of a group of remediation methods thatare often collectively termed enhanced remediation methods, whichtogether form a set of approaches that are more aggressive and typicallymore effective than the conventional methods described above. Thefundamental notion involved with enhanced remediation methods is to takeaggressive actions that markedly affect the mass removal rates of NAPLcontaminated regions, either through increasing rates of mass transfer,Brandes and Farley (1993), or through mobilization, Larson, Davis et al.1982, Roeder, Brame et al. (1996), of previously trapped NAPL residualcontaminants.

CF is implemented by flushing a contaminated region with a mixture ofwater and alcohol(s). Vancho (1994); Imhoff, Gleyzer et al. (1995b). Theflushing solution is designed such that the NAPL contaminant is muchmore soluble in the cosolvent solution than it is in an aqueous solutionalone, increasing the potential rate of removal. If present as asufficient fraction of the flushing phase, cosolvent systems cancompletely solubilize a quantity of NAPL that they come in contactwith—reducing a two-fluid-phase immiscible contamination problem to asingle-fluid-phase miscible displacement problem. Such methods haveworked relatively well in laboratory experiments, and these methods havebeen tried in the field as well. Rao, Annable et al. (1997).

Two potentially significant drawbacks exist for CF approaches: cost andthe heterogeneous distribution of NAPL in the subsurface. Alcohols areexpensive in the quantities typically needed for in situ remediation ofDNAPL contamination. This cost and the need to use relatively highfractions of alcohol for many DNAPL-alcohol combinations of potentialuse presents a considerable economical challenge, which for most caseswill result in the need to recapture and reuse the alcohol solutions.This recapturing process will require the use of some sort of separationfacility to separate contaminants, water, and alcohols into theircomponent parts. The large volumes of alcohols needed and the complexityand expense of their recovery poses significant limitations to thewidespread use of CF.

In addition, as previously described, DNAPL contamination of subsurfacesystems often results in complex patterns of DNAPL distributions,including pooling of DNAPLs on low permeability materials. These poolsattain length scales and distributions that result in accumulation incoarse-grained materials and pore openings that impede the flow ofwater, or a cosolvent mixture, through the DNAPL contaminated region.Under such circumstances, the flushing solution will largely bypass theDNAPL-contaminated zone and mass removal will occur through dissolutionalong the edges of the DNAPL pool. This is another sort of mass transferlimitation which can significantly increase the number of pore volumesof flushing needed to clean up a contaminated region compared to thecase with an equivalent volume of DNAPL that is uniformly distributedthroughout a region. As the number of pore volumes of flushing needed tomeet a remediation target increases, the time and expense involved withCF remediation increases as well. The economics of CF are an importantlimitation and constraint involved with the method.

Surfactant Flushing

Surfactant flushing (SF) is another method in the class of enhancedremediation methods, which is similar in many respects to CF, includingsimilar advantages and limitations. Similar to cosolvents, surfactantscan significantly increase the solubility of a DNAPL in an aqueoussolution and, under certain conditions, can result in the formation of asingle, stable fluid phase that contains a mixture of water, surfactant,and DNAPL. Sharma and Shah (1989); Pope and Wade (1995); Shiau, Sabatiniet al. (1996). Surfactants can significantly reduce the interfacialtension (IFT) that exists between a DNAPL and an aqueous phase, often byorders of magnitude under appropriate conditions. Reductions in IFTreduce capillary forces and encourage the mobilization of DNAPLs thatwere previously held immobile in a state of residual saturation.Pennell, Pope et al. (1996). When such mobilization occurs, rapid andefficient removal of DNAPLs can result—often removing the majority ofDNAPL residual within a few volumes under controlled laboratoryconditions. Willson, Hall et al. (1998).

While surfactant methods show promise, Okuda, Mcbride et al. (1996), anumber of important challenges arise, which have not been routinelyovercome as yet. Pennell, Abriola et al. (1996). First, surfactants areexpensive, so it is important to have a process that is efficient interms of the number of pore volumes that must be flushed. It is alsousually necessary for economic reasons to implement a separationsstrategy to capture and reuse the surfactant solution, which addscomplexity and expense to the use of the process. Lipe, Sabatini et al.(1996). Second, reductions in IFT can mobilize a DNAPL, but if theviscous forces are not sufficient to overcome gravity forces, the DNAPLmay be transported vertically, potentially spreading the contaminationto previously uncontaminated portions of a subsurface system. Longinoand Kueper (1995); Mason and Kueper (1996). Third, phase behavior forsurfactant solutions is affected by a variety of chemical properties,including surfactant fraction in solution, pH, and ionic strength, andcontrolling the set of these factors to affect the desired behavior inthe subsurface can be a challenge. Fourth, heterogeneous distributionsof DNAPLs, especially pools, can lead to flow bypassing and masstransfer limitations that can greatly increase the number of porevolumes of a flushing solution needed to reach a given DNAPL removalfraction.

Steam Injection

Steam injection (IS) is an enhanced remediation process that relies uponthe injection of steam into the subsurface to convert the DNAPL presentto a gas phase, which may be easily removed from the system. Forsyth(1994). This process can be effective at removing entrapped DNAPL, butseveral challenges remain. Sittler, Swinford et al. (1992). The processis typically implemented such that the steam must contact DNAPL, whichsuggest that water and DNAPL in the system are volatilized. As aminimum, the system temperature must be brought to and maintained at theboiling point of the DNAPL until the DNAPL gas has been removed from thesystem. Since subsurface systems tend toward thermodynamic equilibrium,this implies that the water and solid materials present are also heatedto a similar minimum level—either the boiling point of DNAPL or water.The energy required to accomplish this can be substantial, especiallyfor cases in which the DNAPL has penetrated deeply into the saturatedzone. A further complication is that when the DNAPL is heated a varietyof physiochemical properties are altered, including surface tension,density, viscosity, and diffusion coefficients. For regions of largeDNAPL saturation, such changes ahead of a steam front can mobilize aDNAPL by changing the balance of forces and allow the DNAPL to penetratedeeper regions of an aquifer.

Reactive Walls

Reactive walls (RW) are a technology that has evolved rapidly over thelast few years. This technology is implemented by placing a barrier inthe subsurface that directs the groundwater flow through specificregions in the construct. These regions are comprised of an appropriatematerial that facilitates the transformation of contaminants that arepresent in the groundwater phase. Groundwater then exits the wall freeof the target contaminant and not a threat to potential down-gradientusers of the resource. Kaplan, Cantrell et al. (1996). The material thatsuch walls are constructed from varies depending upon the application.Zero-valent iron is a popular choice for sites that are contaminatedwith DNAPLS such as TCE and PCE. In some cases, RW are used in a regionto establish and promote biological growth and transformation of thetarget contaminants of interest.

There are some drawbacks to RW, the most obvious being that the DNAPLcontaminated region is not affected by the method, only the contaminantin the dissolved phase. Because such walls are constructed typicallyunder natural gradient conditions, their period of operation will beaffected by the same set of mass transfer limitations as the PAT method,but over a longer period of time since DNAPL removal rates will be masstransfer and transport limited. Thus, RW for DNAPL contamination must beoperative over periods of time similar to or longer than PAT methods,which can range to times over a century. In addition, RW can beexpensive to construct and difficult to maintain, especially when thepotentially large depths of DNAPL penetration are encountered and thelong periods of maintenance needed are considered.

Because each of these approaches has significant limitations, recentefforts have centered on containment or treatment of the groundwaterthat is contaminated by the DNAPL. However, such efforts are deemedunsatisfactory in that they treat a symptom of the subsurfacecontamination problem, as opposed to the problem itself.

What is needed is a method to effectively, quickly, and economicallyremediate subsurface systems that are contaminated with DNAPLs. Such amethod is currently unavailable in the art.

DISCLOSURE OF THE INVENTION

Disclosed herein is a method for remedying contamination of a subsurfaceenvironment by a non-aqueous phase liquid that is denser than water(DNAPL). The subsurface environment comprises a resident aqueous phaseand a DNAPL phase, and the method includes the steps of introducing tothe subsurface environment a dense aqueous solution that has a densitygreater than the density of the DNAPL phase present in the subsurfaceenvironment; displacing the resident aqueous phase with the denseaqueous solution, thereby causing the DNAPL phase to rise above thegreater density aqueous solution; and recovering the DNAPL phase andthereby substantially remedying contamination of the subsurfaceenvironment.

Also disclosed herein is an alternative method for remedyingcontamination of a subsurface region by a non-aqueous phase liquid(DNAPL) that is denser than water. The subsurface region comprises aresident aqueous phase and a DNAPL phase, and the method includes thesteps of introducing to the subsurface region a dense solution layer(DSL) that has a density greater than the density of the DNAPL phasebeneath the surface region contaminated with the DNAPL phase; dewateringdownwardly to the top of the dense solution layer (DSL) so as to createan unsaturated porous medium above the dense solution layer (DSL) and todownwardly mobilize the DNAPL phase to the top of the dense solutionlayer (DSL); and collecting the DNAPL phase from the top of the densesolution layer (DSL) and thereby substantially remedying contaminationof the subsurface region.

Accordingly, it is an object of the present invention to provide amethod for remedying contamination of a subsurface environment by aDNAPL which provides for effective and economical cleanup of acontamination site.

It is another object of the present invention to provide a method forremedying contamination of a subsurface environment by a DNAPL whichdoes not promote downward migration of DNAPLs.

It is yet another object of the present invention to provide a methodfor remedying contamination of a subsurface environment by a DNAPL whichworks in presence of DNAPL pools.

It is a further object of the present invention to provide a method forremedying contamination of a subsurface environment by a DNAPL whichprovides for rapid cleanup times.

It is still a further object of the present invention to provide amethod for remedying contamination of a subsurface environment by aDNAPL which utilizes a non-toxic and inexpensive flushing solution.

Some of the objects of the invention having been stated hereinabove,other objects will become evident as the description proceeds, whentaken in connection with the accompanying drawings as best describedhereinbelow.

The file of this patent contains at least one photograph executed incolor. Copies of this patent with color photograph(s) will be providedby the Patent and Trademark Office upon request and payment of necessaryfee.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a color photograph of a glass cell packed with sand media andshowing simulated subsurface contamination by a DNAPL; and

FIG. 2 is a color photograph of the cell depicted in FIG. 1 afterintroduction of a dense aqueous solution in accordance with the methodsof the present invention;

FIG. 3 is a color photograph of a glass cell packed with sand media andshowing simulated subsurface contamination by a DNAPL wherein thedarkest media has the largest grain size and wherein a DDIwater-saturated condition has been established;

FIG. 4 is a color photograph of the cell depicted in FIG. 3 after TCEwas added from the top and allowed to reach a stable configuration;

FIG. 5 is a color photograph of the cell depicted in FIG. 3 after a thinbrine layer was established on the bottom of the domain;

FIG. 6 is a color photograph of the cell depicted in FIG. 3 after theupper portion of the cell has been dewatered;

FIG. 7 is a color photograph of the cell depicted in FIG. 3 after a 0.4pore volume pulse of surfactant has been added at the top of the cell;and

FIG. 8 is a color photograph of the cell depicted in FIG. 3 after thesurfactant has mobilized both the DNAPL and the water towards theextraction needle at the bottom of the cell and wherein a largeproportion of the TCE from the system has been removed.

BEST MODE FOR CARRYING OUT THE INVENTION

Contamination of subsurface environments by non-aqueous phase liquidsthat are denser than water (DNAPLs), such as chlorinated solvents, is animportant environmental problem. Efforts to date to remediate suchcontamination have been only partially successful, because of thephysics that controls the movement and trapping of such contaminantsbelow the water table. In accordance with the present invention a novelapproach for remediation of such systems that relies upon reversal ofthe density gradient between the aqueous phase and the DNAPL isprovided. This is accomplished by increasing the density of the aqueousphase by introducing a dense aqueous solution, such as a salt solution,to the subsurface environment and displacing the resident aqueous-phasesolution within the subsurface environment with the dense aqueoussolution. This density gradient reversal effectively floats the DNAPLsto the top of the water table, where they can be easily recovered.

To overcome trapping in fine-grained materials, surfactant solutions canbe added. Optionally, the surfactant solution can be introduced to thesubsurface environment as a first or initial step of the method of thepresent invention. Alternatively, the surfactant solution can beintroduced to the subsurface environment after the introduction of afirst volume of the dense aqueous solution in accordance with the methodof the present invention. After density-enhanced displacement, theremaining small amount of DNAPL present can be readily removed usingconventional enhanced-remediation methods, such as cosolvent orsurfactant flushing, with a relatively small volume of flushingsolution. Density-enhanced remediation of DNAPLs approach is shownherein to be an effective and efficient approach for restoration ofcontaminated subsurface environments.

The method of the present invention particularly addresses the problemsobserved in prior art methods with respect to NAPL entrapment. Toelaborate on these problems, NAPL entrapment results in a staticdistribution of an organic liquid in the subsurface. The static stateresults from the release of DNAPLs into typical subsurface environments.Once present in the subsurface, NAPLs follow complex patterns of flow,which are influenced by the porous medium and fluid properties manifestas gravity, viscous, and capillary forces. Since LNAPLs are less densethan water, they reside in the unsaturated zone and generally above thewater table. DNAPLs tend to move vertically under gravitational forcesand, if present in sufficient quantities to exceed the capillary entrypressure of the saturated zone, they will penetrate the water table.Schwille (1988).

Continuing with the problems associated with NAPL entrapment, capillaryforces trap some fraction of a NAPL in the pore space, leaving behind aresidual saturation present as a phase that may be continuous only overa single pore space or small set of connected pores. The distribution ofresidual NAPL is influenced by the morphology of the pore space, thewettability of the solid phase with respect to the fluids occupying thepore space, the density and interfacial tension of the NAPL phase, thevelocity of the aqueous phase, and the boundary conditions associatedwith the aqueous phase and NAPL release. Pennell, Pope et al. (1996).

The distribution of DNAPL in the subsurface plays a crucial role indetermining the difficulty associated with removing it from thesubsurface. NAPLs present in the unsaturated zone above the water tableare often the intermediate wetting phase, with the gas phase occupyingthe larger pore sizes and water occupying the smallest pore sizes. Belowthe saturated zone interface, DNAPLs are usually assumed to be thenon-wetting phase and tend to occupy the largest pore spaces.

Consider a two-fluid-phase system consisting of an aqueous phase and aDNAPL phase that is non-wetting (i.e. non-aqueous), and a solid phase.In a homogeneous porous media, the pore size distribution is uniform inspace. Since DNAPL residual saturation is often expressed as a functionof funicular DNAPL saturation, homogeneous regions exposed to similarmaximum concentrations of funicular DNAPL saturation would be expectedto have similar levels of residual DNAPL saturation. However, naturalsystems are heterogeneous by nature, many markedly so. In heterogeneoussystems, the largest pore sizes are distributed non-uniformly in thesubsurface, which in turn leads to a non-uniform distribution of DNAPLresidual. In heterogeneous finite entry pressure media, stable DNAPLpools of relatively high saturation can form in coarse-grained regionsof the system. These pools have length scales that are consistent withthe length scales of the porous media, which may be from centimeters tometers. Pools also form in depressions in hydraulic barriers, such asclay layers, that form the bottom boundary of a permeable hydraulicunit. Michalski, Metlitz et al. (1995). Because pools can have highDNAPL saturations, pooled regions are typically relatively impermeableto the flow of the wetting phase, which leads to mass transferlimitations for any method requiring phase change. Because of this, NAPLpools are the dominant feature limiting restoration of a DNAPLcontaminated porous media. Whelan, Voudrias et al. (1994); Pearce,Voudrias et al. (1994).

As detailed above, efforts to remediate DNAPL contaminated subsurfacesystems have generally failed for a number of reasons:

(1) DNAPLs tend to move below the saturated zone and are difficult tolocate;

(2) DNAPLs form pools that can lead to mass transfer limitations anddownward mobilization;

(3) existing methods require many pore volumes of flushing, with theactual number increasing as a function of the heterogeneity of thesystem increases; and

(4) existing enhanced methods of subsurface remediation are expensive.

To overcome these limitations with existing methods, in accordance withthe present invention a novel method of DNAPL remediation of saturatedporous media is provided: density-enhanced remediation of DNAPLs (DERD).The basic notion behind DERD is straightforward: the wetting phasedensity is modified so that trapped DNAPLs become significantly lessdense than the aqueous phase. This leads to gravity forces acting upwardon the DNAPL phase and motivates the upward migration of DNAPL towardthe water table. If the wetting (i.e. aqueous) characteristics of theaqueous phase are not modified, the tendency will be for the DNAPL tomigrate through the larger pore openings, precisely the class of poresizes favored during the downward migration phase that contaminated themedia.

Because DERD is a mobilization approach, mass transfer limitations areavoided. Also, unlike other enhanced remediation methods, mobilizationcaused by DERD must be in an upward direction—avoiding the potentialproblem of contaminating deeper, previously uncontaminated portions ofan aquifer during a restoration effort.

The density modifications of the aqueous phase that are needed can beeasily and economically accomplished using a variety of means, such asthe addition of salt or sugar solutions in either single ormulti-component form to a create a dense aqueous solution. Othermixtures that form wetting phases that are denser than the target DNAPLsare possible as well, but the exact choice of the solution is secondaryand the optimal choice will depend upon local geochemistry conditions,the DNAPL of concern, and economic considerations related to the cost ofthe compounds added to the wetting phase. As an example of densitiesthat are readily achievable, Table 1 lists the properties of somesimple, common, single-component salt solutions. These properties can becompared to properties of some common DNAPLs that are listed in Table 2.This shows that a wide variety of choices exist to achieve densities ofan aqueous solution that are significantly greater than typical DNAPLsand which will accomplish the objective of DERD.

TABLE 1 Densities of Single-Component Salt Solutions temp. g/100 gdensity Name formula (deg. C) sat. soln. g/cm³ ammonium iodide NH₄I 2564.5 1.646 ammonium nitrate NH₄NO₃ 25 68.3 1.320 barium iodideBaI₂—7.5H₂O 25 68.8 2.277 barium bromide BaBr₂ 20 51.0 1.710 bariumchlorate Ba(ClO₃)₂ 25 28.5 1.294 barium chloride BaCl₂ 20 26.3 1.27barium perchlorate Ba(ClO₄)₂ 25 75.3 1.936 calcium bromide CaBr₂ 20 58.81.82 calcium chloride CaCl₂—6H₂O 25 46.1 1.47 calcium iodide CaI₂ 2067.6 2.125 lithium bromate LiBrO₃ 18 60.4 1.830 magnesium MgBr₂—6H₂O 1850.1 1.655 bromide magnesium iodide MgI₂—8H₂O 18 59.7 1.909 potassiumchloride KCl 25 26.5 1.178 potassium citrate KC₆H₅O₇ 25 60.9 1.514potassium iodide KI 25 59.8 1.721 potassium formate KCHO₂ 18 76.8 1.571sodium bisulfate NaHSO₄—H₂O 25 59.0 1.47 sodium bromide NaBr—2H₂O 2548.6 1.542 sodium chlorate NaClO₃ 25 51.7 1.440 sodium chloride NaCl 2526.5 1.198 sodium hydroxide NaOH 25 50.4 1.51 sodium iodide NaI 25 64.81.919 sodium perchlorate NaClO₄ 25 67.8 1.683 sodium tungstateNaWO₄—10H₂O 18 42.0 1.573 sucrose C₁₂H₂₂O₁₁ 25 67.89 1.340

TABLE 2 Densities of Common DNAPLS DNAPL Density carbon tetrachloride1.59 o-dichlorobenzene 1.31 m-dichlorobenzene 1.29 1,1-dichloroethane1.17 1,2-dichloroethane 1.26 1,1,1-trichloroethane (1,1,1-TCA) 1.351,1,2-trichloroethane (1,1,2-TCA) 1.44 1,1-dichloroethylene 1.22trichloroethylene (TCE) 1.46 tetrachloroethylene (PCE) 1.63

Standard Application

As an example, DERD can be implemented in the following fashion:

(1) locate the approximate region of the DNAPL contamination;

(2) bound the region with wells that will be used for injection of adense wetting-phase solution;

(3) install a shallow pumping well in the center of the region whichpenetrates the upper portion of the saturated zone;

(4) begin pumping a rate sufficient to induce a gradient that capturesthe flow from the target region—running the effluent through a suitabletreatment system;

(5) inject a dense wetting-phase solution at the set of bounding wellsat a depth that bounds the vertical extent of the contamination, whichwill result in the displacement of the native water with the injectedsolution;

(6) maintain the pumping and injection pattern until free-phase DNAPL isno longer collected from the pumping well;

(7) discontinue injection of the dense wetting-phase solution andmaintain pumping to capture the residual injected solution and remove itfrom the subsurface; and

(8) remove the remaining residual DNAPL by conventional methods, such asflushing through the installed wells with a surfactant or cosolventsolution.

It is important to note that many modifications of this scheme arepossible within the general concept of density-enhanced displacement.Aspects that might vary include: the method and pattern used to deliverthe dense wetting-phase solution, the method and pattern used towithdraw the flushing and resident fluids (e.g., horizontal or verticalwells, drains), the rates of injection and withdrawal, the preciseproperties of the injected fluid, and the choice and implementation ofschemes to remove the remaining residual left behind after the gravityenhanced displacement process phase. It is also envisioned that vaporextraction might be useful during a DERD process for some applications.

Thus, in application, it is contemplated that the DERD method of thepresent invention can assume a large number of forms, with subsurfaceconditions and other factors affecting sound engineering design at anygiven location. This is the usual case for any remediation methodapplied in the field. Exemplary techniques for applying remediationtechniques in the field can be found in U.S. Pat. Nos. 5,753,122 (IBDprocess) and 5,449,251 (SI process), the entire disclosure of which areherein incorporated by reference.

Heterogeneous Conditions

When subsurface conditions are moderately to highly heterogeneous,modifications to the basic DERD scheme outlined above may prove useful.Under such conditions, trapping of DNAPL can occur by contact withrelatively fine-grained, and small pore size, materials overlying aregion of entrapped DNAPL. That is, as the dense wetting-phase solutioncontacts the trapped and pooled DNAPL, gravity forces will act to causeit to move upward. However, if these forces are insufficient to move theDNAPL through a fine-grain region, because of capillary considerations,the DNAPL can remain trapped. To overcome such situations, the basicDERD scheme outlined above can be modified by following a dense-wettingphase solution injection step with the injection of surfactant solutionfollowed in turn by a second dense wetting phase solution injectionstep. The surfactant will lower the IFT of the DNAPL, allowing it tomove into and through finer grained materials, but it will also bebrought to the surface by gravity forces from the dense wetting-phasesolution that resides below it. Residual DNAPL left in fine-grainedregions after the surfactant and second dense wetting-phase solutionsare flushed from the system can be removed by flushing with aconventional surfactant or cosolvent solution. As before, variousmodifications of this basic scheme are possible.

The following Examples have been included to illustrate preferred modesof the invention. Certain aspects of the following Examples aredescribed in terms of techniques and procedures found or contemplated bythe present inventor to work well in the practice of the invention.These Examples are exemplified through the use of standard laboratorypractices of the inventor. In light of the present disclosure and thegeneral level of skill in the art, those of skill will appreciate thatthe following Examples are intended to be exemplary only and thatnumerous changes, modifications and alterations can be employed withoutdeparting from the spirit and scope of the invention.

EXAMPLES

The DERD method outlined above has been investigated in a set oflaboratory experiments with encouraging results. Briefly, theseexperiments emplaced a dyed TCE solution into a heterogeneous porousmedia under saturated conditions in a thin, essentially two-dimensionalglass cell packed with glass beads. Visual observations of the DNAPLdistribution were possible at all phases of the investigation. Followingemplacement, the aqueous phase was displaced with a dense salt solutionby injecting in an up-flow mode. Rapid and substantial displacement ofthe trapped and pooled DNAPL occurred, with a substantial amount of theDNAPL readily captured at the surface of the cell. This displacementoccurred after a single pore volume of the salt solution was added andcaptured an estimated about 65% of the trapped DNAPL from this highlyheterogeneous system. The remaining portion could have been easilyremoved using a combination of a surfactant followed by a dense saltsolution flush to remove vertically trapped regions of high saturation,and a phase of conventional surfactant or CF.

This Example pertains to the use of a dense brine flushing solution toremove pooled DNAPLS from the subsurface. Experimental parametersincluding the dimensionality, media, packing geometry, and fluids werechosen to represent conditions similar to those commonly encountered inthe field. The results clearly demonstrate the basic removal mechanismof the method of the present invention. Further, the results indicatethat, with some minor modifications within the skill of the art, similardensity—enhanced flushing strategies are feasible for many real-worldDNAPL remediation scenarios.

Experimental Setup

A bench-scale experimental cell was constructed to qualitatively testthe density-enhanced flushing method of the present invention. Thedimensions of the cell and properties of the media and chemicals used inthis experiment are detailed below.

To represent field conditions involving DNAPL pools, an essentiallytwo-dimensional flow cell was chosen rather than common but essentiallyone-dimensional laboratory columns. The construction and geometry of thecell are described in Table 3 and are essentially the same as thatdescribed in Thyrum (1994) and in Imhoff, Thyrum et al. (1996), as it isthe same frame with some slight modifications.

TABLE 3 Details of the Experimental Flow Cell Parameter ValueConstruction: frame aluminum with stainless steel and brass fittingswindows plate glass gasket neoprene rubber screens stainless steelInternal dimensions: sand width 20.0 ± 0.2 cm sand height 15.0 ± 0.2 cmsand thickness  2.0 ± 0.5 mm reservoirs  0.5 × 0.5 × 7.75 in Porousmedia: coarse Quackenbush Company, “Q-bead”, 0.8-mm glass beads fineCataphote MILG-9954A size 4, $>0.25$-mm glass beads very fine CataphoteMIL-G-9954A size 10, 0.125 < d₅₀ < 0.150 mm glass beads

The cell was packed with three different kinds of glass beads. The firstwas a very fine glass bead (size 10) that was used only along the sidesof the cell within one-half inch of the gasket. It was placed there tokeep free-phase DNAPL from contacting and destroying (dissolving) thegasket material. The fine (size 4) glass beads were used to form thebulk of the domain and they correspond to a relatively fine sand.

The third glass bead (nominally 0.8-mm diameter) was used to create thecoarse inclusion. The average size of these beads corresponds to acoarse sand. It was used as it has an appreciably lower entry pressurethan the finer beads and, when used with them, could be expected toreadily form stable TCE pools on the order of 15-cm in height. This isclearly sufficient for the size of the domain chosen.

At the top and bottom of the sand domain, a stainless steel screen wasused in conjunction with stainless steel wool. The screen acted as aporous barrier that would allow fluids to easily pass through whileretaining even the finest of the glass beads. The stainless steel woolwas used as a porous support for the relatively flimsy screen material.A stainless steel needle was inserted through the top screen as a meansof injecting DNAPL into the domain.

The DNAPL and the flushing solutions that were used are described inTable 4. All water used in the experiment was de-ionized and thende-aired (DDI) using a vacuum pump. The trichloroethylene (TCE) used asa DNAPL was an analytical grade reagent and was subsequently dyed withless than 0.01% by weight Oil-Red-O (ORO) to give it a red color andmakes it much more visible in FIGS. 1 and 2 attached hereto. The ORO dyehas little or no effect on the physical properties of the TCE and willnot partition into the aqueous phase.

TABLE 4 Fluid Properties p @ 20° C. μ @ 20° C. Fluid (g/cc) (cp)trichloroethylene (TCE) 1.4642 0.57 DDI water 1.0 1.0 NaI solution (58%by weight NaI) 1.7518 2.095 NaI/Aerosol AY-OT mixture 1.6675 -ND-

Experimental Procedure

The following is a description of the sequence of events. Time-linesthat correspond to still photographs that were taken during theexperiment, as presented in FIGS. 1 and 2, are provided in Tables 5 and6. The times recorded in FIGS. 1 and 2 and in Tables 5 and 6 are“wall-times” in hr:min:sec relative to the time at which DNAPL was firstinjected into the domain.

After creating the packing, air in the cell was displaced with carbondioxide. The cell was then flushed with DDI water in an up-flow fashionto displace and/or dissolve all of the carbon dioxide gas within thecell. This procedure resulted in a domain completely saturated with DDIwater. The cell was then flushed with more than ten pore volumes of DDIwater to clean (rinse) it and ensure that all gas bubbles were gone.

A syringe was used to slowly inject the DNAPL (TCE with ORO) into thedomain. When a pool of desired dimensions had formed, the TCE injectionceased. The TCE was allowed to reach a static fluid configuration byletting it sit overnight.

The next morning, the Nal solution was pumped into the domain in anup-flow configuration. Since the Nal solution was much more dense thanwater (see Table 4), it was a stable displacement scenario and theexperiment clearly demonstrated a sharp and level front between the DDIwater and the advancing flushing front. At the front, a significantfraction of the total TCE mass collected and floated out of the sanddomain. This is clearly shown in the still images presented in FIGS. 1and 2.

When the flush was finished, the TCE removed from the sand was extractedfrom the upper reservoir of the flow cell using a second syringe. Thecell was once again flushed—this time with a down-flow current of DDIwater. The DDI water was used to remove the Nal solution from the sandand to see what would happen to the TCE that remained. The TCE leftwithin the domain (especially in the lower left-hand comer, where themost TCE remained) did shift downwards slightly during the DDI waterflush which indicated that a small TCE pool was still present.

TABLE 5 Sequence of Events - Simulation of Contamination Date/TimeDescription Day 1-Day 3 The “Thyrum” cell was packed with glass beads.The pattern was chosen as it was a “clearly heterogeneous” packing. Therear glass plate cracked after packing (during final tightening of thebolts—gasket compression). To keep the cell from leaking, the crackedrear glass plate was covered by an acrylic (Plexiglass (tm)) insert anda seal was made using Vaspar (a putty-like wax). The cracks are evidentduring the experiments but they are sufficiently tight that the DNAPL isnot able to enter or pass through them. The cell was flushed with CO₂gas and then many pore volumes (>10) of DDI H₂O to dissolve away all gasbubbles and wash the cell and glass beads. Day 5 DNAPL was injected.Initially, the cell is saturated with DDI H₂O and the DNAPL is injected,using a syringe, through the SS needle at the top of the domain. Day 500:00:00 DNAPL (TCE with ORO) injection starts. 00:00:11 ±2.5 mLcumulative DNAPL injected. 00:00:14 ±3.5 mL cumulative DNAPL injected.00:00:15.5 ±4.35 mL cumulative DNAPL injected. 00:00:17 ±5.6 mLcumulative DNAPL injected. 00:00:19 ±6.45 mL cumulative DNAPL injected.00:00:21 ±7.35 mL cumulative DNAPL injected. 00:00:22 DNAPL injectioncomplete: ±7.35 mL total. The DNAPL clearly formed a single pool of high(>50%) saturation with some residual DNAPL trapped near the point ofinjection.

TABLE 6 Sequence of Events - Remediation of Contamination Date/TimeDescription Day 6 DNAPL is allowed to reach static conditions overnight.Cell leaked slightly more than 10 mL of water over night. However, noair entered the domain due to the constant-head water reservoirsconnected at both ends. Day 7 22:15 Brine solution is injected into thedomain from the bottom using a constant-head marriot. Brine solutionfirst fills the lower reservoir (8.0 by 0.5 by 0.5 in) and thedifference in densities is clearly visible due to diffraction. 22:22Brine begins to enter the glass beads from the bottom and is visible asit darkens the domain—NaI solution is clear yellow in color. 22:24 Brinefront has moved half-way through the domain. 22:26 Brine reaching top oftop of glass beads and TCE flotation is clearly evident. 22:27 All theDNAPL that is removed by flotation is, at this point, removed from theglass beads. 22:28-22:34 NaI solution fills the upper reservoir (8.0 by0.5 by 0.5 in). 22:50-23:54 DNAPL is removed from the domain using asyringe. Unfortunately, a mass balance was not possible due toevaporative and other losses during the DNAPL extraction procedure.23:56 Start of reverse flush of DDI water to remove brine solution.24:02 Roughly half of the brine solution has been flushed from thedomain. 24:15 Essentially all of the brine solution has been flushedaway. Figure 2 shows DNAPL entrapment caused by the geometry of thepacking.

From observation and image analysis of the bench-scale flow experimentperformed during the period described in Tables 5 and 6 above, thefollowing results are noted.

(1) Density-enhanced approaches can remove a significant fraction of theDNAPL present in pools. Image analysis showed that roughly 65% of theTCE was removed in this experiment.

(2) Density-enhanced approaches can accomplish rapid removal—only onepore volume of flushing solution is required.

(3) While capillary trapping may limit the effectiveness of this method,the addition of a surfactant could drastically reduce trapping andresult in a much higher percent removal.

DNAPL contamination of subsurface systems is an important problem thatremains largely unsolved. Existing methods to remediate subsurfacesystems suffer from a variety of limitations, which in most cases areamplified by heterogenous conditions that exist in virtually all naturalsystems. A new method, density-enhanced remediation of DNAPLs (DERD), isintroduced as an alternative to existing technologies. A standardapproach and some typical modifications to the DERD approach areoutlined in sufficient detail to allow the implementation of the methodat the field scale by one skilled in the art. DERD is an effective andeconomical method for restoration of DNAPL-contaminated subsurfacesystems, and some of the relative advantages over prior art alternativetechniques are set forth in Table 7 below.

TABLE 7 Overview of Cleanup Strategies - Strengths and Weaknesses PAT(1) not likely to promote downward migration of DNAPLs (2) highoperating and maintenance costs (3) very slow—essentially perpetualtreatment required CF (1) potentially rapid cleanup of some sites (2)may cause further contamination by downward migration (3) expensiveflushing solution (4) potentially toxic flushing solution SF (1)potentially rapid cleanup of some sites (2) may cause furthercontamination by downward migration SI (1) potentially rapid cleanup ofsome sites (2) unsuited to some (especially deep) spill scenarios (3)expensive process (large energy input) RW (1) low operating andmaintenance costs (2) unsuited to some (especially deep) spill scenarios(3) very slow—essentially perpetual treatment required DERD (1)potential for rapid cleanup of all sites (2) not likely to promotedownward migration of DNAPLs (3) works in presence of DNAPL pools (4)fast cleanup times (5) non-toxic flushing solution

Alternative Embodiment of Invention

As discussed previously, restoration of environmental systemscontaminated with hazardous wastes is a costly problem, averaging about$9 million per site in 1996 with an expected $750 billion in restorationremaining to be accomplished over the next 30 years in the United Statesalone. Oppelt (1999). Contamination of subsurface environments bynon-aqueous phase liquids that are denser than water (DNAPLs), such aschlorinated solvents, is an important subset of this overall restorationproblem because such contaminants are believed to be a significant riskto human health when present at relatively low concentrations in theaqueous phase. Remediation of DNAPL-contaminated systems is among themost difficult problems in the broad field of environmental sciences,because DNAPLs are long-lived in the subsurface environment andextremely difficult to remove—so difficult that some researchers haveopined that the removal of DNAPL sources is essentially a hopelessproblem.

Vigorous research efforts over the last two decades have continued inpursuit of an effective strategy. These efforts have included flushing acontaminated system with water, water-alcohol mixtures, water-surfactantmixtures, steam, and a gas phase. Significant efforts have also beenexpended to understand and promote biological transformations of DNAPLs,such as the chlorinated solvents tetrachloroethylene andtrichloroethylene. Despite all of these efforts, an efficient and safeapproach for removing DNAPLs; that is, one requiring relatively few porevolumes of an inexpensive flushing solution and presenting limited riskfor spreading the contamination to previously uncontaminated portions ofthe system, has yet to be advanced and applied except as described andclaimed herein.

The second approach proposed by applicant's invention is as follows:

1. a dense solution layer (DSL) is established underlying a regioncontaminated with a DNAPL;

2. the DNAPL-contaminated region is dewatered down to the top of theDSL, creating an unsaturated porous medium above the DSL, which istermed the primary phase of treatment;

3. DNAPL that is mobilized downward is collected from the top of theDSL;

4. if needed, one or more pulses of a flushing solution, such as asurfactant or cosolvent, is flushed through the unsaturated zone and theflushing solution and any mobilized DNAPL is collected from the top ofthe DSL, which is termed the secondary phase of treatment; and

5. a tertiary phase of treatment can optionally be applied to remove anyremaining DNAPL from the unsaturated zone, which might, for example,consist of vapor extraction.

The rationale with this approach is that the establishment of a DSL willprovide a vertical barrier to restrict the movement of DNAPL intouncontaminated regions of the system. Upon dewatering, gravity forcesacting on the DNAPL will be increased, resulting in a significantfraction of DNAPL migration in the vertical direction but only down tothe DSL where it can be collected. Pulsing the unsaturated zone with aflushing solution, such as a surfactant, provides a means tosignificantly reduce the interfacial tension between the NAPL andaqueous phase, further mobilizing DNAPL down to the DSL where it can becollected. Because of interfacial tension reductions, such secondarytreatment pulses can also result in the removal of water from the porespace, which has advantages for certain tertiary treatment phases. Thesepulses could be effective even when much less than a pore volume inmagnitude. Under some situations where difficult conditions exist or avery high level of removal is desired, a tertiary treatment phase can beapplied. For sufficiently volatile contaminants, vapor extraction can bean appealing approach.

Many different solutions are possible to form the DSL including a widerange of salts, formate solutions, acetate solutions, and other fluidphases that have a density greater than that of the DNAPL. The actualsolution used in any application will depend mostly upon the DNAPLcharacteristics, local geochemical conditions, and supplier costs. Theselection of this solution is considered a typical aspect of siteengineering, which is a part of any subsurface remediation method.

An important variant of this approach that will be effective undercertain hydrogeolocial conditions is to dewater the unsaturated zone,but not establish a DSL. Instead, a natural (e.g., silt, clay, or otherlow permeability material) or engineered barrier (e.g., slurry wall)could form an impermeable lower boundary and serve to confine thevertical migration of the DNAPL. Under such conditions, no DSL would beneeded. Instead, the DNAPL could be collected via a layer of similarthickness to the DSL but consisting of a surfactant or cosolventsolution. The key concepts of the technology of mobilizing the DNAPLvertically by creating unsaturated conditions, pulsing a flushingsolution through the unsaturated zone to encourage additional removal,collecting the DNAPL through a confined lower barrier and mobile fluidphase, and perhaps performing vapor extraction as a tertiary phase allwould remain the same under this approach.

Similar comments apply to the specification of the flushing solutionused for the secondary treatment phase. Surfactants and cosolvents, suchas water-alcohol mixtures, form two broad classes of approaches withmany possible solutions existing from within these classes. Otherflushing solutions and enhancements, such as thermal modification andsteam injection, are possible. The key feature is that the secondarytreatment is applied to the unsaturated media in such a way thatprimarily vertical flow occurs and the intent is to remove DNAPL trappedin the pore space and deliver it to the top of the DSL.

The above approach will also work as well for less-dense than waternon-aqueous phase liquids (LNAPLs), such as petroleum products. For thiscase, the density of the aqueous phase may be sufficiently high toimpede the vertical migration of the NAPL phase, so the establishment ofa DSL may not be necessary for some cases. Secondary and tertiary phaseswould apply for the case of LNAPLs in a manner analogous to theapplication for DNAPLs.

EXPERIMENTAL INVESTIGATION Experimental Setup

To investigate the practicality of this approach, a bench-scaleexperimentation cell was assembled to quantitatively test the new idea.The dimensions of the cell and properties of the media and chemicalsused in this experiment are detailed below.

To represent field conditions involving DNAPL pools, an essentiallytwo-dimensional flow cell was chosen rather than a common butessentially one-dimensional laboratory column. The construction andgeometry of the cell is described in Table 8 and is very similar to thatdescribed by Thyrum (1994) and Thyrum et al. (1996) as it is the sameframe with some slight modifications.

The cell was packed with three different sands. The fines sand (F-52)was used only along the sides of the domain to act as a capillarybarrier to hinder the DNAPL (TCE) from contacting the gasket material.This was done to prevent the TCE from swelling, rupturing, or otherwisecompromising the gasket between the glass and aluminum frame.

The medium (C778) and coarse (Accusand 20/30) sands were packed in alayered configuration. The roughly horizontal layers were chosen tomimic conditions in many sandy stratified-drift aquifers. Thesecontinuous layers can act as capillary barriers that promote theformation of DNAPL pools. Given the differences in pore sizes and theproperties of the DNAPL (TCE), these two sands can result in a maximumpool height of approximately 12 cm.

At the top and bottom of the sand domain, a stainless steel screen wasused in conjunction with stainless steel wool. The screen and wool actedas a porous barrier that allowed fluids to easily pass through the cellwhile retaining even the finest of the sands. The stainless steel woolwas used as porous support for the relatively flimsy screen material.

TABLE 8 Details of the Experimental Flow Cell Parameter ValueConstruction: frame aluminum with stainless steel and brass fittingwindows plate glass gasket neoprene rubber screens stainless steelInternal Dimensions: sand width 20.0 ± 0.2 cm sand height 15.0 ± 0.2 cmsand thickness  2.0 ± .5 mm reservoirs  1.27 × 1.27 × 19.7 cm PorousMedia: coarse Accusand 20/30, 0.713 ± 0.023 mm medium Sand, C778, 0.355mm fine Sand, F-52, 0.255 mm

Two needles were inserted into the domain. The lower needle protruded upthrough the screen and was placed there to act as an extraction pointduring the desaturation process. The upper needle was inserted in asimilar fashion and was used for the injection of the DNAPL.

The DNAPL and the flushing solutions that were used are described inTable 9. All water used in the experiment was de-ionized and thende-aired (DDI) using a vacuum pump. The trichloroethylene (TCE) used asa DNAPL was an analytical grade reagent and was subsequently dyed withless than 0.01% by weight Oil-Red-O (ORO) to give it a red color thatwas more visible to the camera. Past experience has shown that the OROdye has little or no effect on the physical properties of the TCE andwill not partition into the aqueous phase.

TABLE 9 Fluid Properties p @ 20° C. μ @ 20° C. Fluid (g/cc) (cp)trichloroethylene (TCE) 1.4642 0.57 DDI water 1.0 1.0 NaI solution (60%by weight NaI) 1.7957 2.10

Experimental Procedure

After creating the packing, air in the cell was displaced with carbondioxide. The cell was then flushed with DDI water in an up-flow fashionto displace and/or dissolve all of the carbon dioxide gas within thecell. This procedure resulted in a domain completely saturated with DDIwater. The cell was then flushed with more than ten pore volumes (>2 L)of DDI water to rinse the domain and ensure that all gas bubbles weredissolved.

The upper syringe was used to slowly inject the DNAPL (TCE with ORO)into the domain. When a total of 13.5 g had been transferred, the TCEinjection ceased. The TCE was allowed to reach a static fluidconfiguration by letting it redistribute for a few hours.

Next, the Nal solution was pumped into the domain in an up-flowconfiguration. Since the Nal solution was more dense than water (seeTable 9), it was a stable displacement and the experiment clearlydemonstrated a sharp and level front between the DDI water and theadvancing flushing front.

Following the Nal injection, a syringe pump was used to extract fluidfrom the lower needle. This extraction was done at a rate of 1.0 ml/minand continued even when the DNAPL and air entered the syringe. When theinitial desaturation had reached a point where mostly air was withdrawn,some surfactant solution was added. This surfactant was a 1% solution ofa 1:1 by weight mixture of sodium diamyl-and sodiumdioctyl-sulfosuccinates (SDSS). Properties of this surfactant mixturewere detailed by Willson et al. (1999). When the surfactant addition wascased and the rate of fluid extraction had become very slow (mostly gasextraction was occurring), the pump was turned off and the experimentceased.

FIGS. 3-8 shows a sequence of images from this experiment, where thedarkest media has the largest grain size. The images describe: a DDIwater-saturated condition was established (FIG. 3); TCE was added fromthe top and allowed to reach a stable configuration (FIG. 4); a thinbrine layer was established on the bottom of the domain (FIG. 5); theupper portion of the cell was dewatered (FIG. 6); a 0.4 pore volumepulse of surfactant was added at the top of the cell (FIG. 7); and thesurfactant mobilized both DNAPL and water towards the extraction needle(FIG. 8). The results from this experiment clearly show that a largefraction (>90%) of the TCE from this heterogenous system was removed.Further, the addition of the surfactant resulted in water removal aswell, leaving the small remaining fraction of TCE in a state ofrelatively large surface area volume and low water saturation, which isan ideal condition for removal of the remaining fraction of thisvolatile contaminant via gas-phase extraction.

Results

From observations of the bench-scale flow experiment applicant reportsthe following results:

Density-enhanced approaches can remove a significant fraction of theDNAPL present in pools. Image analysis showed that over 90% of the TCEwas removed in this experiment.

Density-enhanced approaches can accomplish rapid removal.

The addition of a surfactant can clearly improve DNAPL removalefficiency.

The dense brine solution acted as an effective lower barrier, preventingthe migration of the DNAPL out of the sand domain.

Summary

Remediation of DNAPLs and LNAPLs is an important environmental concern,for which effective and economical solution strategies have previouslynot been known. Applicant has discovered a class of three-fluid-phaseapproaches, including several variants, for mobilizing and capturingboth DNAPL and LNAPL contaminants, that have been shown to be highlyeffective and economical.

References

Ahlfeld, D., Dahmani, A., Hoag, G., Farrel, M., and Ji, W. (1994),Proceeding of the Petroleum Hydrocarbons and Organic Chemicals in GroundWater: Prevention, Detection, and Remediation Conference, 175-190.

Bartow, G. and Davenport, C. (1995), Ground Water Monitoring andRemediation, 15(2):140-146.

Bass, D. H. and Brown, R. A. (1995), Proceeding of the 1995 PetroleumHydrocarbons and Organic Chemicals in Ground Water: Prevention,Detection, and Remediation, 621-636.

Brandes, D. (1992), Effect of phase Behavior on Residual DenseNonaqueous Phase Liquid Displacement from Porous Media by AlcoholFlooding, Ph.D. Thesis, Clemson University, Clemson, S.C.

Brandes, D. and Farley, K. J. (1993), Journal of Water EnvironmentResearch, 65(7):869-878.

Cherry, J. A., Feenstra, S., Kueper, B. H., and McWhorter, D. W. (1990),International Specialty Conference on How Clean is Clean? CleanupCriteria for Contaminated Soil and Groundwater, Air and Waste ManagementAssociation.

Cherry, J. A., Feenstra, S., and Mackay, D. M. (1992), SubsurfaceRestoration Conference—Third International Conference on Water QualityResearch, 14-17.

de Marsily, G. (1986), Quantitative Hydrogeology: Groundwater Hydrologyfor Engineers, Academic Press, San Diego, Calif.

Domenico, P. A. and Schwartz, F. W. (1990), Physical and ChemicalHydrogeology, John Wiley and Sons, New York.

Dupont, R. R. (1993), Environmental Progress, 12(1):45-53.

Forsyth, P. A. (1994), International Journal for Numerical Methods inFluids, 19(12):1055-1081.

Imhoff, Gleyzer et al. (1995), Environmental Science & Technology,29(8):1966-1976.

Imhoff, P. T., Thyrum, G. P., Miller, C. T. (1996), Water ResourcesResearch, 32(7):1929-1942.

Kaplan, Cantrell et al. (1996), Journal of Environmental Quality,25(5):1086-1094.

Larkin and Hemingway (1991), Chemicals in Ground Water: Prevention,Detection, and Restoration, 191-204.

Larson, R. G., Davis, H. T., and Scriven, L. E. (1982), Journal ofPetroleum Technology, 243-258.

Lipe, Sabatini et al. (1996), Journal of Petroleum Technology, 243-258.

Longino, B. L., and Kueper, B. H. (1995), Canadian Geotechnical Journal,32:296-308.

Mackay, D. M., and Cherry, J. A. (1989), Environmental Science &Technology, 23(6):630-636.

Marley, M. C. (1991), Proceedings of the Fifth National Outdoor ActionConference on Aquifer Restoration, Ground Water Monitoring, andGeophysical Methods, 89-102.

Mason, A. R., and Kueper, B. H. (1996), Environmental Science &Technology, 30(11):3205-3215.

Mayer and Miller (1996), Water Resources Research, 32(6):1551-1567.

Mccann, M., Boersma, P., Danko, J., and Guerrier, M. (1994),Environmental Progress, 13(3):208-213.

Mercer and Cohen (1990), Journal of Contaminant Hydrology, 6(2):107-163.

Michalski, Metlitz et al. (1995), Ground Water Monitoring andRemediation, 15(1):90-100.

Miller, Poirier-McNeill et al. (1990), Water Resources Research,26(11):2783-2796.

Okuda, I., Mcbride, J., Gleyzer, S. N., and Miller, C. T. (1996),Environmental Science & Technology, 30(6):1852-1860.

Oppelt, E. T. (1999), Technical Report EPA/600/R-98/154.

Pankow, J. F., and Cherry, J. A. (1996), Dense Chlorinated Solvents andOther DNAPLs in Groundwater: History, Behavior, and Remediation.

Pearce, Voudrias et al. (1994), Journal of EnvironmentalEngineering—ASCE, 120(5):1191-1206.

Pennell, K. D., Abriola, L. M., and Loverde, L. E. (1996), The use ofsurfactants to remediate NAPL-contaminated aquifers, L. N. Reddi,editor, Non-Aqueous Phase Liquids (NAPLs) in the Subsurface Environment:Assessment and Remediation, Proceedings of the Specialty Conference Heldin Conjunction with the ASCE National Convention, 221-232, New York,American Society of Civil Engineers.

Pennell, K. D., Pope, G. A., and Abriola, L. M. (1996), EnvironmentalScience & Technology, 30:1328-1335.

Pope and Wade (1995), Lessons from Enhanced Oil Recovery Research forSurfactant-enhanced Aquifer Remediation, Sabatini, Knox et al., eds.,Surfactant-Enhanced Subsurface Remediation: Emerging Technologies,142-160. American Chemical Society, Washington, DC.

Rao, P. S. C., Annable, M. D., Sillan, R. K., Dai, D., Hatfield, K.,Graham, W. D., Wood, A. L., and Enfield, C. G., (1997), Water ResourcesResearch, 33(12):2673-2686.

Roeder, E., Brame, S. E., and Falta, R. W. (1996) Non-Aqueous PhaseLiquids (NAPLs in Subsurface Environment: Assessment and Remediation),L. N. Reddi, ed., 333-344, New York.

Ross and Lu (1994), Water Resources Research, 30(2):581-584.

Schwille, F. (1988), Dense Chlorinated Solvents in Porous and FracturedMedia.

Semprini, Roberts et al. (1990), Ground Water, 28(5):715-727.

Sharma and Shah (1989), Use of Surfactants in Oil Recovery, Donaldson,Chilingarian, eds., Enhanced Oil Recovery II: Processes and Operations,255-315, Elsevier, Amsterdam.

She, Sleep et al. (1995), Journal of Soil Contamination, 4(3):227-242.

Shiau, Sabatini etal. (1996), Environmental Science & Technology,30:97-103.

Sittler, S. P., Swinford, G. L., and Gardner, D. G. (1992), Proceedingsof the 1992 Petroleum Hydrocarbons and Organic Chemicals in GroundWater: Prevention, Detection, and Restoration, 413-426. The AmericanPetroleum Institute and The Association of Ground Water Scientists andEngineers (a division of NGWA).

Thyrum, G. P. (1994), An Investigation of Residual Nonaqueous PhaseLiquid Dissolution in Water Saturated Porous Media. Master of ScienceThesis, University of North Carolina, Chapel Hill, N.C.

Travis and Macinnis (1992), Environmental Science & Technology, 26(10):1885-1887.

Tsien, Brusseau et al. (1989), Applied and Environmental Microbiology,55(12):3155-3161.

U.S. Army Corps of Engineers (1995), Soil vapor extraction andbioventing, Engineering and Design EM 1110-1-4001, U.S. Army Corps ofEngineers, Washington, DC.

U.S. Pat. No. 5,449,251

U.S. Pat. No. 5,753,122

Vancho, L. A. (1994), The cosolvent effect of methanol on theequilibrium between tetrachloroethylene and water, Master thesis,University of North Carolina at Chapel Hill, Chapel Hill, N.C.

Walter, G. R. (1994), Ground Water, 32(1):146-148.

Ward, C. H., Cherry, J. A., and Scalf, M. R. (1997), SubsurfaceRestoration.

Whelan, Voudrias et al. (1994), Journal of Contaminant Hydrology,15(3):223-237.

Willson, C. S., Hall, J. L., Imhoff, P. T., and Miller, C. T. (1998),Factors affecting bank formation during surfactant-enhanced mobilizationof residual NAPL, Environmental Science and Technology.

Windholz, M. (1983), Merck Index, Merck & Co., Rahway, N.J.

Zacharias, Lang et al. (1995), Water Research, 29*7):1663-1671.

It will be understood that various details of the invention may bechanged without departing from the scope of the invention. Furthermore,the foregoing description is for the purpose of illustration only, andnot for the purpose of limitation—the invention being defined by theclaims.

What is claimed is:
 1. A method for remedying contamination of asubsurface region by a non-aqueous phase liquid (DNAPL) that is denserthan water, the subsurface region having a resident aqueous phase and aDNAPL phase, the method comprising the steps of: (a) introducing a densesolution layer (DSL) that has a density greater than the density of theDNAPL phase beneath the subsurface region contaminated with the DNAPLphase; (b) dewatering downwardly to the top of the dense solution layer(DSL) so as to create an unsaturated porous medium above the densesolution layer (DSL) and to downwardly mobilize the DNAPL phase to thetop of the dense solution layer (DSL); and (c) collecting the DNAPLphase from the top of the dense solution layer (DSL) and therebysubstantially remedying contamination of the subsurface region.
 2. Themethod of claim 1, wherein the DNAPL is selected from the groupconsisting of carbon tetrachloride, o-dichlorobenzene,m-dichlorobenzene, 1,1-dichloroethane, 1,2-dichloroethane,1,1,1-trichloroethane (1,1,1-TCA), 1,1,2-trichloroethane (1,1,2-TCA),1,1-dichloroethylene, trichloroethylene (TCE), tetrachloroethylene (PCE)and combinations thereof.
 3. The method of claim 1, wherein the densesolution layer is selected from the group consisting of a salt solution,a formate solution, an acetate solution, a sugar solution orcombinations thereof.
 4. The method of claim 3, wherein the densesolution layer comprises a component selected from the group consistingof ammonium iodide, ammonium nitrate, barium iodide, barium bromide,barium chlorate, barium chloride, barium perchlorate, calcium bromide,calcium chloride, calcium iodide lithium bromate, magnesium bromide,magnesium iodide, potassium chloride, potassium citrate, potassiumiodide, potassium formate, sodium bisulfate, sodium bromide, sodiumchlorate, sodium chloride, sodium hydroxide, sodium iodide, sodiumperchlorate, sodium tungstate, sucrose, and combinations thereof.
 5. Themethod of claim 1, wherein the resident aqueous phase and the DNAPLphase define a first volume in the subsurface environment, and whereinthe dense aqueous solution is introduced in a volume substantiallyequivalent to the first volume defined by the resident aqueous phase andthe DNAPL phase in the subsurface environment.
 6. The method of claim 5,further comprising removing any remaining DNAPL phase from theunsaturated porous medium by vapor extraction.
 7. The method of claim 1,further comprising: (a) introducing one or more pulses of a flushingsolution through the unsaturated porous medium; and (b) collecting theflushing solution and any additional mobilized DNAPL phase from the topof the dense solution layer (DSL) and thereby substantially remedyingcontamination of the subsurface environment.
 8. The method of claim 7,wherein the flushing solution comprises a surfactant or cosolventsolution.
 9. A method for remedying contamination of a subsurface regionby a non-aqueous phase liquid (DNAPL) that is denser than water, thesubsurface region having a resident aqueous phase, a DNAPL phase, andone of a natural or engineered barrier layer beneath the residentaqueous phase and the DNAPL phase, the method comprising the steps of:(a) dewatering downwardly to the top of the barrier layer so as tocreate an unsaturated porous medium above the barrier layer and todownwardly mobilize the DNAPL phase to the top of the barrier layer; and(b) collecting the DNAPL phase from the top of the barrier layer andthereby substantially remedying contamination of the subsurface region.10. The method of claim 9, wherein the DNAPL is selected from the groupconsisting of carbon tetrachloride, o-dichlorobenzene,m-dichlorobenzene, 1,1-dichloroethane, 1,2-dichloroethane,1,1,1-trichloroethane (1,1,1-TCA), 1,1,2-trichloroethane (1,1,2-TCA),1,1-dichloroethylene, trichloroethylene (TCE), tetrachloroethylene (PCE)and combinations thereof.
 11. The method of claim 9, wherein the barrieris a natural barrier selected from the group consisting of silt, clay orother fine soil type.
 12. The method of claim 9, wherein the barrier isan engineered barrier comprising a slurry wall.
 13. The method of claim9, further comprising: (a) introducing one or more pulses of a flushingsolution through the unsaturated porous medium; and (b) collecting theflushing solution and any additional mobilized DNAPL phase from the topof the barrier layer and thereby substantially remedying contaminationof the subsurface region.
 14. The method of claim 13, wherein theflushing solution comprises a surfactant or cosolvent solution.
 15. Themethod of claim 13, further comprising removing any remaining DNAPLphase from the unsaturated porous medium by vapor extraction.
 16. Amethod for remedying contamination of a subsurface region by anon-aqueous phase liquid that is less dense than water (LNAPL), thesubsurface region having a resident aqueous phase and a LNAPL phase, themethod comprising the steps of: (a) introducing a dense solution layer(DSL) that has a density greater than the density of the LNAPL phasebeneath the subsurface region contaminated with the LNAPL phase; (b)dewatering downwardly to the top of the dense solution layer (DSL) so asto create an unsaturated porous medium above the dense solution layer(DSL) and to downwardly mobilize the LNAPL phase to the top of the densesolution layer (DSL); and (c) collecting the LNAPL phase from the top ofthe dense solution layer (DSL) and thereby substantially remedyingcontamination of the subsurface region.
 17. The method of claim 16,wherein the dense solution layer is selected from the group consistingof a salt solution, a formate solution, an acetate solution, a sugarsolution or combinations thereof.
 18. The method of claim 17, whereinthe dense solution layer comprises a component selected from the groupconsisting of ammonium iodide, ammonium nitrate, barium iodide, bariumbromide, barium chlorate, barium chloride, barium perchlorate, calciumbromide, calcium chloride, calcium iodide lithium bromate, magnesiumbromide, magnesium iodide, potassium chloride, potassium citrate,potassium iodide, potassium formate, sodium bisulcate, sodium bromide,sodium chlorate, sodium chloride, sodium hydroxide, sodium iodide,sodium perchlorate, sodium tungstate, sucrose, and combinations thereof.19. The method of claim 16, further comprising: (a) introducing one ormore pulses of a flushing solution through the unsaturated porousmedium; and (b) collecting the flushing solution and any additionalmobilized LNAPL phase from the top of the dense solution layer andthereby substantially remedying contamination of the subsurfaceenvironment.
 20. The method of claim 19, wherein the flushing solutioncomprises a surfactant or cosolvent solution.
 21. The method of claim19, further comprising removing any remaining LNAPL phase from theunsaturated porous medium by vapor extraction.
 22. A method forremedying contamination of a subsurface region by a non-aqueous phaseliquid that is less dense than water (LNAPL), the subsurface regionhaving a resident aqueous phase and a LNAPL phase, the method comprisingthe steps of: (a) lowering the water table so as to create anunsaturated porous medium above the aqueous phase and to downwardlymobilize the LNAPL phase to the top of the aqueous phase layer; and (b)collecting the LNAPL phase from the top of the aqueous phase and therebysubstantially remedying contamination of the subsurface region.
 23. Themethod of claim 22, further comprising: (a) introducing one or morepulses of a flushing solution through the unsaturated porous medium; and(b) collecting the flushing solution and any additional mobilized LNAPLphase from the top of the aqueous phase and thereby substantiallyremedying contamination of the subsurface environment.
 24. The method ofclaim 23, wherein the flushing solution comprises a surfactant orcosolvent solution.
 25. The method of claim 23, further comprisingremoving any remaining LNAPL phase from the unsaturated porous medium byvapor extraction.